IndVarSimplify's design is somewhat odd in the way how it reports that
some transform has made a change. It has a Changed field which can
be set from within any function, which makes it hard to track whether or
not it was set properly after a transform was made. It leads to oversights
in marking it properly, see example in PR38855.
This patch removes the Changed field, turns it into a local and unifies
the signatures of all relevant transform functions to return boolean value
which designates whether or not this transform has made a change.